1. Field of The Invention
The invention relates to thermoplastic molding compositions and more particularly to molding compositions which are formulated from polyester-carbonate resins.
1. Brief Description of the Related Art
Polyester-carbonates [also known as "copoly(ester-carbonates)" or "polyester-polycarbonates"] are a well known class of thermoplastic resin as is their preparation; see for example the descriptions given in U.S. Pat. Nos. 3,169,121 and 4,487,896.
Polyester-carbonate (PEC) resins have been useful thermoplastic molding resins, to mold particular articles of certain physical properties. In general, the polyester-carbonate resins have had limited use for molding some articles, being characterized as yielding molded articles having little resistance to impact when subjected to testing for notched Izod impact strengths.
In the U.S. Pat. No. 4,393,161 to Van Abeelen et al (Jul. 12, 1983) there is a suggestion that by incorporating fibrous reinforcement materials (such as glass fibers) into polycarbonate resins, including polyester-carbonate resins, one can improve certain physical properties of articles molded therefrom. For example, elastic modulus is dramatically improved. However, it is said, that "this always results in a serious deterioration in overall ductility, manifested in poor notched and unnotched impact strength". It is then related that if one adds a conventional impact modifier to the blend, "there is a detrimental effect on stiffness (modulus) strength". To improve impact strength with a polystyrene resin impact-modifier and "almost full retention of the elastic modulus" Van Abeelen et al teaches the use of unsized glass fibers as the reinforcement, and the addition of minor proportions of poly C.sub.1 -C.sub.10 alkyl (or phenyl) hydrogen siloxanes "having a substantial content of Si-H bonds". We have confirmed the teachings of Van Abeelen et al., in respect to polycarbonate homopolymers. However, we have now found that particular polyester-carbonate resin based molding blends containing fibrous reinforcement and a so-called "conventional" impact modifier (not necessarily a polystyrene resin), can be used to thermally mold articles exhibiting acceptable elastic modulus and high impact strength (with other desired properties), without addition of the above-described polysiloxane ingredient. The improvement is particularly advantageous in respect to aromatic polyester-carbonate resins having aliphatic diester chain units. This improvement is unexpected, in view of the prior art teachings and further does not detract from flame retardance when flame retardants are also added to the blends.